The Kumeyaay were the most resistant of all the California tribes to the conversion efforts of the Spanish priests. Following the founding of the Mission in San Diego in 1769, it was over a year before the Mission had its first converts.

Spanish weaponry and armor made the soldiers virtually invincible to the wood and stone weapons of the Kumeyaay. Nevertheless, the Kumeyaay launched repeated attacks on the Spanish throughout their 52 year occupation. The most famous of these was the attack and destruction of the San Diego Mission in 1775. Because of this resistance the Spanish seldom controlled more than a 30 mile strip of the coast.

In 1821, following the successful Mexican revolution, California became part of Mexico. The Mexican government sought to eliminate the Spanish system centered on the Missions and Pueblos.

Lands were carved up for distribution as Ranchos and Indians were either evicted or forced to work as laborers. This resulted in a massive uprising of Kumeyaay throughout their territory. Armed with modern weaponry and horses, Kumeyaay warriors launched recurring raids on the Mexican Ranchos. By 1842, the Ranchos had been abandoned and the warriors were attacking the last stronghold, the City of San Diego. The City was spared destruction by the entry of another faction, the United States of America.

In 1846, General Kearney led his forces to San Diego. Although offered allegiance by Kumeyaay in Santa Ysabel, Kearney only asked that the Kumeyaay stay out of the battle. He promised fairness for the Indians under the United States. The Mexican-American War ended in 1848 and the border was drawn through the heart of Kumeyaay lands. In 1852, the Kumeyaay Kwa-pai met in Santa Ysabel and negotiated a treaty with the United States. This treaty was the mechanism whereby the Kumeyaay people acknowledged their status as a nation within a nation.

Unfortunately, the Treaty of Santa Ysabel was illegally and unethically voted down and placed under seal by the Senate of the United States. State sponsored militias then sought to enslave or exterminate all Indians in California. The population of Indians in California dropped by 90% from 1850 to 1860. Because of the nearby Mexican border and the lack of large gold strikes to lure more Americans, the Kumeyaay fared somewhat better than tribes further north. This did not prevent the break up of the Kumeyaay territories into fragmented parcels, however.

In 1875, the first of these parcels began to be converted to Reservation trust land. Further additions were taken into trust over the next 25 years including the first portion of the Campo Indian Reservation in 1893. The Campo Valley was known as Meelqsh G'tay (or big open medow) and was know by the local non-Indians as Milguatay. Eventually, it was translated into the Spanish word for field or country.